402 BBC build

Hello All. New to this forum. Hopefully someone can help me with my 402 rebuild.

I am restoring my 1970 GMC sierra grande truck. I am currently going thru the engine. The indication is that is came with a 396 but i understand they marked the 402 as 396 also during this time. I have the engine apart and have done some research and found this is a true 402 block 0.040 over bore to 4.166. I want to get about 400+ hp when done. i think I can stay with my block/heads/crank. I want a good street rod and occasional play at the track but mainly street. Here is what i think I know.

Block:Engine ID - T0406TBGCasting - 39698542 boltThis seems to be a 402 block built at the Tonawanda plant April 6th 1970 for a truck. Could pass as original equipment I believe but has been rebuilt at some time.

Heads:Casting - 3931063Open chamber2.06/1.72 valvesFrom my research I have seen some info that indicates 96.4cc and some 100.9cc. Not sure which is correct.

Crank:Casting - 3904816Rod journals at least have been undersized 0.010.

All info on heads that I have found indicate closed chamber but these seem clearly to be open chamber. I have found a lot of reference material and none seem to agree with cc size. I'll have to measure it.

I have decided on using KB16-60 Hypereurectic pistons. 1.765 compression height .180 dome with the stock 6.135 rods and stock 3.76 crank. This with the COMP CAM 11-209-3 .507/.531 lift and 219/229 duration with 1.72:1 rockers, flat tappet hydraulic lifters and appropriate springs. From many hours calculating I believe I will have about about at least 0.08" intake valve clearance and over 0.1" exhaust valve clearance I hope. Of course I will verify these numbers upon assembly. From what I have calculated I hope to get 440hp @ 4000rpm. This is with a 500-550 cfm carb. I think about 9.84:1 compression.

I could not find any info on how to pick a CAM and piston that would work before purchase so I put together a spreadsheet with my own calculations.

Could someone verify my numbers and possibly give some advice before I drop money on parts?

The open chamber big chevy heads use a much larger chamber. Its about chamber CC. The open chamber heads are in the 120cc range, where the closed chamber heads are around 100cc. Your heads are indeed closed chamber heads, and are actually very good heads I might add.

Usually chamber volumes vary on factory production heads. Lots of factory open chamber heads are listed at 118cc, yet you find they often measure out around 122 in some cases. Maybe they werent super consistant in the way they were cast at the factory....or maybe its just from 40 years of the seats sinking and what not. I suppose it just depends on what the head has been through over the years. You will have to measure to be dead sure where they fall. Somewhere around 100 im sure but you wont be positive til you check.

Id prefer a forged piston anytime, but if you want to stay hyper, that KB piston will work well. If you did change your mind, make SURE you are looking at closed chamber pistons. The dome shape will vary. You need a closed chamber piston just to be totally sure there will be no interference with your chambers in your head.

The current piston you are looking at fits the bill. If you stick with KB be totally sure to follow their instructions on ring gap. Their top rings on the hyper pistons are located a shade higher than other pistons and they need a wider ring gap than other pistons. You dont want rings butting so follow their instructions.

The cam isnt a bad choice, but 440 horsepower is asking a lot. With very good head work, the mild camshaft will have a better chance of getting the job done, but with a mostly stock valve size, stock as cast port head...I believe 375-400 is more likely. Also 4000 rpm is VERY early for the small big block. Peak torque will be closer to 4000. Peak horsepower would probably fall around 5400ish with that camshaft.

Also, why such a small carb? Bare minimum use a 650, however Id prefer a 750.

Your current build is similar to my 408 cube big block in a lot of ways. I have a shade more camshaft and a 750 HP holley(mechanical secondary), with 215 heads(virtually a clone of yours) and it likes to go 6000rpm.

With only a few minor differences I believe mine might make 420 at best but its been on a dyno. Soon I'll be stuffing in a roller with some head work to go along with it and "hope" to end up with a solid 450 horsepower.

The cam will be decent, but the little carb will hurt. The build as a whole will be more on the mild side as is but still have a nice sounding idle and good power.

Depending on what you want...giving it just a little more cam and definitely more carb might be a consideration. How fast would you wanna go should you ever race it?

Me being new to big blocks I read some books that explained the difference between open and closed chambers. The closed chambers had chambers wrapped right around the valves and the open chambers spread out around the plug. Mine are not wrapped around the valve so my assumption was open chambers. Again my research from different sites gave me a wide range on the cc's so i will have to measure them for sure.

I have tried several forums trying to get suggestions on CAM lift and timing that I would be comfortable with spending money on without wasting time and money finding out that it didn't have the clearance with no responses. I sat down and put all the calculations in a spreadsheet to graph clearances because of my inexperience to determine a reasonable clearance. I tried several CAMs and without knowing the piston dimensions I guessed from my numbers on what would work. Apparently asking advice about CAMs is a no no I have found from responses.

Again the reason for the carb selection is that I had a 750 on it before I tore it down and I had, what I thought was significant, unburnt fuel and plug fouling. The research I did gave me a formula to calculate the max carb needed and i came up with 500cfm.

I am trying to do the research and reading a lot. I have built many engines in my youth but all stock rebuilds. This is my first that I want to get performance out of. I want good power after owning a vette, it was great. It's a truck so I don't expect to be great off the line but want good power once going for sure. As far as racing I would just take to the track for fun now and again to just get a reference point.

Me being new to big blocks I read some books that explained the difference between open and closed chambers. The closed chambers had chambers wrapped right around the valves and the open chambers spread out around the plug. Mine are not wrapped around the valve so my assumption was open chambers. Again my research from different sites gave me a wide range on the cc's so i will have to measure them for sure.

I have tried several forums trying to get suggestions on CAM lift and timing that I would be comfortable with spending money on without wasting time and money finding out that it didn't have the clearance with no responses. I sat down and put all the calculations in a spreadsheet to graph clearances because of my inexperience to determine a reasonable clearance. I tried several CAMs and without knowing the piston dimensions I guessed from my numbers on what would work. Apparently asking advice about CAMs is a no no I have found from responses.

Again the reason for the carb selection is that I had a 750 on it before I tore it down and I had, what I thought was significant, unburnt fuel and plug fouling. The research I did gave me a formula to calculate the max carb needed and i came up with 500cfm.

I am trying to do the research and reading a lot. I have built many engines in my youth but all stock rebuilds. This is my first that I want to get performance out of. I want good power after owning a vette, it was great. It's a truck so I don't expect to be great off the line but want good power once going for sure. As far as racing I would just take to the track for fun now and again to just get a reference point.

I really appreciate your input.

having fouled plugs does not mean the carb is too big,,,, I would say out of adjustment or in need of rebuild would be more the problem,,,,?

I would not use any less than 750 cfm on a bbc,,, I use quadrajets on all my big blocks and they run great.

the smaller carb would definitely kill any higher rpm ability under load.

cam selection,,,,,,,,,,, ok are u gonna use the truck like a truck? or like a car and just use it as a cruiser?

reason I ask is lumpy cams make for hassle in a truck that is intended for pulling and heavy loads.

Sounds good I'll stay with a 750cfm. I replaced the 750 with a 650 quad i had original with the truck. Rebuilt it them and will need it again since been sitting for 7 years. That will save me money on another one.

The truck is one that I am restoring for just play around with. Not going to be a truck really at all other than appearance. Wanted to just drive around on weekends and such. Want to sound good not real lumpy but wanted to hear the cam somewhat maybe. I was going to get a 3 piece timing chain cover for easy cam swap maybe to play around and experiment.

There is always the possibility that your chambers have had a little work done to unshroud the valves. Im not sure the pics you are looking at VS your chambers, but some of the closed chamber heads especially need help in the valve shrouding area. so yeah...somebody may have just laid it back a little so that the edge of the chamber isnt cramping the valve.

this is more prevalent when installing the larger 2.19/1,88 valve combo, which may be the diff here. You might have seen some pics with the bigger valve combo and saw how they were a little more cramped up against the edge of the chamber vs your stock 2.06 combo.

Either way though...thats a closed chamber deal.

As far as a camshaft goes....if I were camming a high performance street 402 thats also meant to be driven.....Id put in a lunati voodoo 268 and call it a day. If you had the coin....a hydraulic roller would be in the cards. Similar spec to that voodoo or a shade less duration at .050 but more lift is something id be looking for.

As far as plug fouling goes.... carb size alone isnt the only culprit. You can pretty much take any carb size you want and tweak the idle circuit enough to do whatever you want.

Granted a smaller carb will often have a leaner calibration, but that doesnt mean a 750 cant be tweaked. Sometimes one step hotter spark plugs is all you need to get rid of occasional fouling issues at idle. Good tweaking of the idle mix screws can go a long way also. Proper fuel pressure and float setting come into play also. If you have to dig deeper....tweaking in on the idle feed restrictors and possible idle air bleeds tweaks will also help. Some holleys are notorious for being hard on gas, but with proper care in the right areas.....you can make idle and cruise circuits lean as can be, yet keep full throttle as fat as you want for the most performance. Holleys are the carb im most familiar with, and they are divided up into a series of "circuits". Any of which can be tweaked to provide best of both worlds performance.

The quadrajet as mentioned is a really good carb for this kind of app also with just minor tweaking.

Lastly....those carb formulas you probably found are a little stingy.

as far as picking a carb size....I bet I know what formula you are using, but its also a little dated and doesnt quite compensate. Read this info below. Its on the pro systems website and reflects the new line of thinking that corresponds better to what we have learned over the years. Direct quote from pro systems:

I'm sure you remember this old formula:

CID x RPM x V.E. / 3456 = CFM

Well that formula is still being quoted by magazines and companies etc...but times have changed and carburetors are operating on almost immeasurable amounts of vacuum. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers so we aren't looking to match cfm requirements with cfm ratings.

20.4 = 1.5 hg.

You can see that going from 10 inches of water as a requirement at launch to only 3 inches as a requirement really allows a serious increase in cfm size. This removal of restriction really pays off in cylinder head flow numbers and hp of course. Imagine altering this upstream restrictor when flowing your heads.

Because, most of you have specific application designs, a custom shop/unit is typically the plan.

In the future, use this calculation as a general rule on a modified carburetor:

I email Lunati and they suggested the 60202 voodoo cam. 262/268 .530/.542 which appears to be your suggestion also. I ran this cam in my simulation and I agree it does look better. I have been trying to convince myself to spend the extra $ and go roller. Still debating with myself.

I looked at the heads again and was going to attach a pic but can't seem to figure out how to do that here. Regardless I will take your advice.

CID x RPM x V.E. / 3456 = CFM

This is the exact formula I was using. I'll put it in file 13 now and use yours. I have used holley several times in the past and just have not had much luck with them. Probably because I don't know what I'm doing. I have rebuilt many quads without any problems but seems like I can't get the holleys to work well. Have to do some research and learn more about them. A friend of mine has used a Barry Grant and has been pleased with it.

Yes that looks like an 063 chamber. Of those early closed chamber heads, not all had the exact same shape chamber. Chamber volume was about the same between all of them, but some had more or less issues with valve shrouding because of the chamber shape.

Those look pretty similar to something like a 781 or 049 combustion chamber....but dont have quite the same volume. Plus you figure in the grand scheme of things, your talking about a 20 or so cc difference in a chamber thats already around 100cc. So, with a somewhat similar outer contour....that small size difference doesnt jump right out at you, making one look radically different than another. They do look comparable though.

As far as your valves.....white exhaust valve is normal. The exhaust valve runs hotter. See the intake valve has the benefit of being cooled every time fresh(cooler) fuel and air is pulled into the chamber. Albeit...its cooled mostly from the backside, but its still cooled.

The exhaust valve doesnt have the same luck. Its subject to the heat of combustion, and then all those hot gasses working on the front side and back side of the valve once it opens, but never gets the same cooling effect that the intake valve sees. So to see them turn white is common enough.

If anything, it was slightly lean. But...with either a good Air Fuel ratio logger, or just some good ol fashioned spark plug reading(and looking at the piston tops for color) you can get a good idea of how close the mix was.

Looking at the location of the discoloration/blueing on the spark plug ground strap will give you a good idea if timing was about where it should be also.

Hard to see whats going on with your intake valve. I dont know if the picture is playing tricks on my eyes or what. It looks like you have carbon buildup going on, which is also normal. Your going to end up with deposits on the valve from the fuel, but that cooler valve isnt going to be super heated as much to burn all that stuff off. It just gets baked on.

It almost looks like the valve is hollow or has depth in the pic. Just looks a little weird and hard to see how normal it all looks.

You plan on re-using the heads as is?

last thought, if you use the smaller voodoo 262, I wouldnt go over 9.5:1 compression with the iron heads.